Multi-photon excitation using ultrafast lasers is a common technique for generating 3D images of biological tissue with a spatial resolution in the sub-micron range. A usual laser source for this application is a tunable Ken-lens mode-locked Ti:Sapphire laser with an average output power between 1 and 3 Watts (W) at a pulse-repetition frequency (PRF) of about 80 Megahertz (MHz) and a pulse duration between about 70 and 140 femtoseconds (fs). The typical tuning range of such a laser covers a wavelength range between about 680 nanometers (nm) and about 1080 nm. Extension of the tuning range into the infrared (up to 2 um) can be accomplished by using the output of the laser to pump an external optical parametric oscillator (OPO).
One significant disadvantage of a Ti-Sapphire laser is that it must be pumped at a wavelength of 532 nm using a frequency doubled diode-pumped solid-state (DPSS) laser with an output power in a range between about 5 W and 20 W to generate the gain in the Ti:Sapphire gain-medium of the laser and to achieve an output power of greater 1 W. Such a frequency-doubled DPSS laser is relatively expensive, and can have a cost comparable to the Ti:Sapphire resonator.
There is a need for a more cost effective ultrafast laser source that is tunable over several hundred nm in the visible and infrared (IR) and can deliver pulses having a duration of 100 fs or shorter. One possible approach is to use a laser having a gain-medium that has a wide gain-bandwidth, for example 10 nm or greater that can be pumped by standard diode-lasers and is power-scalable to several Watts of output power. Ytterbium (Yb) doped gain-media in bulk or fiber form meet these requirements. However, while output powers exceeding 100 W range have been achieved with 9XX-nm diode-pumped mode-locked Yb-doped fiber MOPAs (master-oscillator power-amplifiers) and Yb-doped solid state thin-disk lasers, it is technically difficult to achieve sub-100 fs pulse durations at 80 MHz PRF at these power levels.
FIG. 1 is a plot summarizing published average-power and pulse-duration results for mode-locked lasers including various bulk Yb-doped gain-media. Results for media in disk form are indicated by a letter D beside symbols identifying the gain-media. Yb-doped yttrium aluminum garnet (Yb:YAG), yttrium-doped potassium gadolinium tungstate (Yb:KGW), yttrium-doped potassium yttrium tungstate (Yb:KYW), yttrium-doped potassium lutetium tungstate (Yb:KLuW), yttrium-doped lutetium oxide (Yb:Lu2O3), and yttrium-doped lutetium scandium sesquioxide (Yb:LuScO3). Other less commonly used materials are collectively summarized under the symbol for Yb:XXX. Pulse repetition frequencies of the examples are in a range between about 60 and 100 MHz. It can be seen from the plot that the gain-media in thin-disk form provide the highest powers, but, in general, it can be seen that whatever the form of the gain-medium, higher power is accompanied by longer pulse-duration.
Pulses having a duration of less than 100 fs have been demonstrated using mode-locked Yb-doped fiber ring-lasers. However, these fiber ring-lasers use nonlinear polarization rotation as the mode-locking mechanism, and this mechanism is very sensitive to environmental changes and is not suitable for lasers which will be used in a commercial environment. Yb Fiber MOPAs and oscillators have also achieved sub 100 fs pulse durations, but due to strong nonlinear effects in the fiber amplifiers, the pulse shapes of these lasers usually comprise undesirable sidelobes.
In order to provide a cost effective tunable ultrafast laser it will be necessary to overcome above discussed scaling problems of mode-locked Yb-doped laser sources.